This invention relates to a superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to an improved and simplified positive retraction assembly for cryogenic thermal joints useful in MRI magnets.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the coil (for a period, for example, of only ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils even alter power is removed due to the absence of electrical resistance at the superconducting temperature, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
Liquid helium has been used to provide the cryogenic temperatures necessary for superconducting magnet operation and is satisfactory for MRI operation. However, helium is commercially obtained only in a few locations, and the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. This has led to considerable effort being directed at superconducting magnet structures which can be rendered and maintained in superconducting operation with a minimized requirement to continually replace boiled-off helium. Considerable research and development effort has also been directed at the use of mechanical cryocoolers to provide conduction cooling in place of liquid helium cryogen cooling. However, the cooling capability of current cryocoolers is not as great in magnitude of cooling, or in the cooling temperature obtainable, as compared with liquid helium cooling. This increases the need for good thermal insulation and prevention of thermal leakage in such superconducting magnets.
This has in turn led to the continuing need for improved thermal joints to prevent thermal resistance when securing devices together in a superconducting magnet. U.S. Pat. No. 5,247,800, of Michael T. Mruzek, Phillip Eckels and Clyde Gouldsberry, issued Sep. 28, 1993, and assigned to the same assignee as the subject invention discloses the use of soft indium material such as on one member in contact with a raised pattern (such as a knurl or spiral pattern) on the other of the members to be joined. The raised pattern penetrates the soft indium and establishes very good thermal contact between the joined members such as the joining of a cryocooler to the magnet assembly. In fact, the contact is so good that the joints have a tendency to stick or "cold weld" over a period of time such that it becomes extremely difficult to later separate the devices when necessary for repair, replacement or servicing.
It is known to utilize a bellows in combination with such thermal joints. The bellows is compressed during assembly and uniting of the devices. It later transmits a separation force when the fasteners uniting the devices are loosened. However, the bellows force is not reliable in breaking thermal joints which have been in place for a period of time because of the aforementioned sticking or cold welding; the thinwalled bellows being incapable of transmitting substantial force.
Accordingly, there exists a need for an improved mechanism to provide positive retraction for separation of such thermal joints, preferably a mechanism which can supplement the bellows force. However, it is also important that any retraction mechanism minimizes heat leakage which results from connecting devices having portions at different temperatures, and to thermally shield the mechanism from surroundings areas of higher temperature. Moreover, the retraction mechanism must operate in a vacuum environment, excluding the ambient or surrounding air until after the thermal joints are broken to preclude a sudden temperature surge which could upset the thermal balance of the superconducting magnet sufficient to quench the magnet and cause a sudden discontinuance of superconducting operation. That is, it is important that the retraction mechanism enable continued superconducting operation of the magnet during removal of the cryocooler from the magnet assembly.
In addition, there are a number of other generic problems encountered in the operation of superconducting magnets at superconducting temperatures, including problems of differential thermal expansion and contraction of materials, of minimizing cost, and of handling the forces generated by the significant magnetic fields required. All of these overlapping, and at times conflicting, requirements must be satisfied for a practical and satisfactory MRI superconducting magnet structure.